Process for removing residual spin solvent from a gel spun filament, the filament, multi-filament yarn and products comprising the filament

ABSTRACT

Processes for removing residual spin solvent from a gel spun, UHMwPE filament having an effective diameter of above 16 μm, comprise the steps of removing residual spin solvent from the filament to a level of below 100 ppm at elevated temperature, while keeping the filament taut. Gel spun UHMwPE filaments having an effective diameter of above 16 μm and a residual spin solvent residue of less than 100 ppm are also provided. Preferably the filament has a creep rate, measured at 50° C., under a load, so that the initial stress is 600 MPa, of less than 5·10 −6  sec −1 .

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of commonly owned U.S. Ser. No.12/593,469, filed Dec. 7, 2009, (now U.S. Pat. No. 9,260,801), which isthe national phase application under 35 USC §371 of PCT/EP2008/002298,filed Mar. 21, 2008, which designated the U.S. and claims priority toEuropean Application No. 07006238.5, filed Mar. 27, 2007, the entirecontents of each of which are hereby incorporated by reference.

FIELD

The invention relates to a process for removing residual spin solventfrom a gel spun, ultra high molecular weight UHMwPE (UHMwPE) filament,the filament as well as a multi-filament yarn and products for medicalapplications containing the filament.

BACKGROUND AND SUMMARY

A process for producing gel spun, UHMwPE filaments is for exampledisclosed in PCT/NL2003/000872.

It is also known to use gel spun, UHMwPE multi-filament yarn in productsfor medical applications. Examples of such products are sutures andcables. Good examples of such cables include a trauma fixation cable, asternum closure cable, a prophylactic or periprosthetic cable, a longbone fracture fixation cable, a small bone fracture fixation cable. Gelspun UHMwPE multi-filament yarns are suitable for use in suchapplications, since they have favorable mechanical properties like ahigh modulus and a high tensile strength. A gel spun filament howeverhas the draw back that without special treatment it contains residualspin solvent. Although the level of residual spin solvent normally isvery low, it is still high enough to make the filament less suitable foruse in medical applications like sutures and cables, since the residualspin solvent may cause unwanted reactions of the human or animal body,like for instance inflammations. Therefore special treatments arerequired to decrease the level of residual spin solvent in the filamentto a very low level, at least below 100 ppm, preferably however to astill lower level.

Special treatments to remove residual spin solvent include extendedextraction or heating of the filament. Problems that occur because ofsuch treatments are that filament properties are adversely affected, ifa residual spin solvent level of at most 100 ppm is obtained, but evenmore if very low levels of residual spin solvent are obtained, below 80ppm or even further down. This is especially true if filaments with ahigh diameter are used, since removal of solvents by extraction orevaporation becomes increasingly difficult with increasing diameter.However the use of filaments having a high diameter is advantageous,since such filaments are easier to produce. Furthermore, filamentshaving a high diameter are more robust during handling (for example withregard to friction) by a surgeon and more abrasion resistant. One of theproperties that is adversely affected is the tensile strength of theyarn. This is important since the filaments are mostly applied inproducts that have to withstand a high tension. Furthermore thesmoothness of the surface of the filament may be adversely affected, sothat the coefficient of friction of the filament increases. This makesit for example more difficult to stitch a wound with a suture comprisingthe filament. Also the creep rate of the filament increases. A low levelof creep is especially important if the filament is used under tensionfor a long period at elevated temperature in the body, for example as asternum closure or a bone fracture fixation cable.

It is therefore an object of the present invention to provide a processfor removing residual spin solvent from a gel spun UHMwPE filamenthaving an effective diameter of at least 16 μm, which process provides afilament that not only contains a low level or even no measurable amountof spin solvent residues, but also does not show one or more of theabove-identified problems.

This object is achieved according to the invention with a process forremoving residual spin solvent from a gel spun UHMwPE filament having aneffective diameter of above 16 μm, characterised in that the processcomprises the step of: removing residual spin solvent from the filamentto a level of below 100 ppm at elevated temperature, while keeping thefilament taut.

IN WO 2005/066401, examples 29 and 30 a process is disclosed forspinning an UHMwPE filament, which filament contains less than 100 ppmresidual spin solvent. However the filament has a very low diameter ofbelow 8 μm and no special process step for removing residual spinsolvent is disclosed.

With the process of the present invention an UHMwPE filament is obtainedthat has a very low or no measurable amount of solvent residues and yetshows favourable properties. The UHMwPE filaments have a high tensilestrength and a low creep rate. Furthermore the filaments have a smoothsurface. Therefore the filaments are very suited for use in products formedical applications.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a representative plot showing the creep rate as a logarithmicfunction (y-axis) of the total creep deformation (x-axis) in percent.

DETAILED DESCRIPTION

The process for making a gel spun, UHMwPE filament according to theinvention comprises the steps of

-   a) spinning a filament or a plurality of filaments from a solution    of UHMwPE in a spin solvent;-   b) cooling the filaments obtained to form gel filaments;-   c) removing part of the spin solvent from the gel filaments;-   d) drawing the filaments in at least one drawing step before, during    or after removing part of the spin solvent;-   e) removing the residual spin solvent to a level of below 100 ppm.

The filament used in the process according to the invention may compriseany of the known solvents for gel spinning of UHMwPE. Suitable spinsolvents include for example paraffin's, like paraffin oil and paraffinwax, mineral oil, kerosene, decalin, tetralin, toluene, lower n-alkanes,for example hexane, xylene, paraxylene, squalane, cyclo-octane.Preferably decalin is used. Cooling of the filament into a gel filamentmay be performed with a gas flow, or by quenching the filament in aliquid cooling bath. The removal of part of the spin solvent in step c)may be performed by known methods, for example by evaporating arelatively volatile solvent, or by using an extraction liquid.Preferably decalin is used and removed by evaporating.

The process for making an UHMwPE filament comprises drawing the filamentin at least one drawing step. Drawing, that is elongating the filament,generally results in at least partial orientation of the polymermolecules and in better mechanical properties of the filaments. Drawingmay be performed on a semi-solid or gel-like filament, after coolingdown the filament to below the gelation temperature of the solution, oron a solid filament after cooling and removal of solvent. Preferably,drawing is performed in more than one step, e.g. on filaments in geland/or solid state, and/or at different temperatures.

Preferably the filament used in the process according to the inventioncontains at the start of the final step (e) of removing the residualspin solvent a level of residual spin solvent of between 100 and 2000ppm (parts per million by weight). In this way the final filament may beobtained with very low levels of residual spin solvent, still showingfavourable properties. More preferably at the start of the step (e) ofremoving the residual spin solvent the level of residual spin solvent inthe filament is below 1500 ppm, even more preferably below 1000 ppm.Preferably at the start of the step of removing the residual spinsolvent the filament contains a level of residual spin solvent of morethan 200 ppm, even more preferably more than 300 ppm.

Surprisingly it was found that keeping the filament taut in the step ofremoving the residual spin solvent, a superior filament was realised. Bykeeping the filament taut is herein meant that a tension in the lengthdirection of the filament is present. The tension may be applied simplyby winding the filament, preferably as a multi- or mono-filament yarntaut around a frame. When because of the elevated temperature in thestep of removing residual spin solvent the filament yarn tends toshrink, further tension may be build up in the filament. It is alsopossible to actively apply a tension to the filament, for example byconnecting the two ends of the filament to a device and applying a forceto the filament by pulling at one or both ends of the filament or bymoving the filament through an inline treatment station under tension.The required and the optimum tension depend on the actual design of thetreatment station and for the individual design it may be determinedempirically by trial and error. When the filament is treated when tautaround a frame, it is preferred that the tension is sufficient toprevent the filament to slide on the frame during treatment. In oneembodiment, the tension should be above 0.15 g/dtex and preferably thetension should be above 0.30 g/dtex. The tension should be sufficientlylow to prevent damage of the filament and the frame. The maximum allowedtension depends to a very large extent on the design of the treatmentstation and may be determined empirically by trial and error.

Preferably the temperature in the step of removing residual spin solventis above 80° C., more preferably above 90° C., even more preferablyabove 100° C. Preferably the temperature in the step of removing thespin solvent is below 140° C., more preferably below 130° C., even morepreferably below 125° C.

Preferably the step of removing residual spin solvent is carried out inan environment having an oxygen content of less than 3 mol/m³, morepreferably of less than 2 mol/m³, more preferably of less than 1 mol/m³,even more preferably of less than 0.5 mol/m³, most preferably of lessthan 0.2 mol/m³.

In one preferred embodiment the step of removing residual spin solventis carried under a reduced air pressure, so that the environmentcontains above-referred contents of oxygen.

In another preferred embodiment the step of removing residual spinsolvent is carried out by subjecting the filament to super critical CO₂extraction. This is because the process runs fast, very low levels ofresidual spin solvent can be obtained and the properties of the filamentremain at a very high level. Especially good results are obtained bysubjecting the filament to super-critical CO₂ extraction at atemperature between 80 and 147.5° C. and a pressure between 50 and 400bar, more preferably between 110 and 130° C. and a pressure between 100and 400 bar.

In this way a filament is obtained that still shows very good mechanicalproperties and surface smoothness of the filaments and yet containing avery low level of spin solvent. Still further lowering the spin solventcontent, while still obtaining a filament with the desired properties ispossible by extending the time of the critical CO₂ extraction stepand/or by choosing a temperature closer to 130° C. Therefore thetemperature is preferably chosen between 120 and 130° C. The timeinterval for the filament treatment may simply be determined by takingsamples at different time intervals during the extraction process,determining the level of residual spin solvent and comparing thedetermined level of residual spin solvent with the desired level.Typical extraction times vary between half an hour and 24 hours,depending on desired level of residual spin solvent, temperature andfurther process parameters.

Preferably the time interval for the filament treatment and thetemperature are chosen in a way that the final residual spin solventcontent is less than 80 ppm, more preferably below 60 ppm, still morepreferably below 40 ppm, most preferably below 30 ppm.

The process according to the present invention is applied to a filamenthaving an effective diameter above 16 μm. Within the context of thepresent application, the effective diameter is understood to be theaveraged maximum cross-sectional dimension of the filaments. Forfilaments having a circular cross-section and an effective diameter of16 μm the titer is about 1.96 dtex. Preferably the filament has a titerof between 2-100 dtex, more preferably between 2-30 dtex, such as 2-5dtex or even 2-3 dtex. In another embodiment, the effective diameter ofthe filament has an effective diameter of above 18 μm and preferably thefilament has an effective diameter of above 25 μm, such as an effectivediameter of above 75 μm. The higher thickness of filaments are typicallyutilised for mono-filament applications whereas lower diameterstypically would be utilised in multi-filament yarns.

Preferably in the process according to the invention the filament doesnot contain spin finish. It is possible that the spin finish, ifpresent, is removed from the gel spun UHMwPE filament, before the stepof removing the residual spin solvent is carried out. Spin finish may beremoved by the process described in PCT/NL2003/000872. In that caseresidual spin solvent is removed fast and to a very low level, whilemaintaining the properties of the filament.

In the process according to the invention the filament is preferablypresent as a multi-filament yarn. It is however also possible that thefilament is present in a product containing the multi-filament yarn, forexample a suture, a cable, a weft or any further article, provided thatthe filaments are kept taut.

In another embodiment of the process according to the invention, thefilament is a mono-filament. Monofilaments are preferred by surgeons insome cases where compactness is more important than flexibility, forexample for some sutures.

The invention also relates to a gel spun UHMwPE filament having adiameter of above 16 μm and a level of residual spin solvent of lessthan 100 ppm. The invention also relates to a multi-filament yarncomprising such a filament as well as a mono-filament consisting of onesuch a filament.

In one embodiment, the filament has a creep rate at 50° C., measured byapplying a load to the filament so that the initial tensile stress inthe filament is 600 MPa, of less than 5·10⁻⁶ sec.⁻¹, more preferably ofless than 8·10⁻⁶ sec.⁻¹, most preferably of less than 10⁻⁷ sec.⁻¹.

Preferably, the UHMwPE applied in the filament is a linear UHMwPE, i.e.an UHMwPE with less than one side chain or branch per 100 carbon atoms,and preferably less than one side chain per 300 carbon atoms, a branchgenerally containing at least 10 carbon atoms. The UHMwPE may furthercontain up to 5 mol % of or more alkenes that can be copolymerized withit, such as propylene, butene, pentene, 4-methylpentene or octene.

Preferably, the UHMwPE has an intrinsic viscosity (IV) of more than 5dl/g. The IV is a measure for the weight average molecular weight of theUHMwPE. Filaments made from such UHMwPE have very good mechanicalproperties, such as a high tensile strength, modulus, and energyabsorption at break. More preferably, an UHMwPE with an IV of more than10 dl/g is chosen. Such gel-spun UHMwPE filaments offers a combinationof high strength, low relative density, good hydrolysis resistance, andexcellent wear properties, making it suited for use in variousbiomedical applications, including implants. The IV is determinedaccording to method PTC-179 (Hercules Inc. Rev. Apr. 29, 1982) at 135°C. in decalin, the dissolution time being 16 hours, with DBPC as theanti-oxidant in an amount of 2 g/l solution, and the viscosity atdifferent concentrations is extrapolated to zero concentration.

Preferably the gel spun UHMwPE filament according to the inventioncontains less than 80 ppm residual spin solvent, more preferably lessthan 60 ppm, still more preferably less than 40 ppm, most preferablyless than 30 ppm.

The invention also relates to a multi-filament yarn, containingfilaments according to the invention.

Preferably the gel spun, UHMwPE multi-filament yarn according to theinvention has a tenacity of at least 3.4 N/tex, more preferably at least3.6 N/tex, even more preferably 3.8 N/tex, most preferably 4.0 N/tex.

The invention also relates to products for medical applicationcomprising the gel spun UHMwPE multi-filament yarn or monofilamentaccording to the invention. Good examples of such products includesutures and cables, but also endless loop products, bag-like,balloon-like products and other woven and/or knitted products. Goodexamples of cables include a trauma fixation cable, a sternum closurecable, and a prophylactic or per prosthetic cable, long bone fracturefixation cable, small bone fracture fixation cable. Also tube-likeproducts for ligament replacement are considered.

EXAMPLES Preparation of Yarn

A commercial grade Dyneema™ SK 75, delivered by DSM Dyneema, theNetherlands, was washed several times with soaps and solvents, until nospin finish could be detected any more.

The amount of residual spin solvent was about 400 ppm. The yarn wastreated at elevated temperature to remove spin solvent (step e)) asindicated below.

Measurement of Tensile Strength

Tensile properties: tensile strength (or strength), tensile modulus (ormodulus) and elongation at break (or eab) are defined and determined onmultifilament yarns with a procedure in accordance with ASTM D885M,using a nominal gauge length of the fibre of 500 mm, a crosshead speedof 50%/min and Instron 2714 clamps, of type Fibre Grip D5618C. On thebasis of the measured stress-strain curve the modulus is determined asthe gradient between 0.3 and 1% strain. For calculation of the modulusand strength, the tensile forces measured are divided by the titre, asdetermined by weighing 10 metres of fibre; values in GPa are calculatedassuming a density of 0.97 g/cm³. Strength of braided members wasmeasured (after tying one simple knot) on a Zwick 1435 apparatus withInstron 1497 K clamps.

Measurement of Creep Level

The creep is measured by placing a piece of the yarn in a hot air ovenat 50° C., connecting one end of the yarn by winding it several timesaround a round bar, having a diameter of 10 mm and connecting the otherend with a weight, so that at the start of the creep measurement thelevel of tensile stress is 600 MPa.

The creep behaviour is represented in a plot in which the creep rate isgiven as a logarithmic function (y-axis) of the total creep deformation(x-axis) in %. In such a plot 3 regions can be observed, as indicated inFIG. 1. Region I (indicated by I in FIG. 1) is finished after arelatively short time and is characterized in a step decrease of thecreep rate. Region II (indicated by II in FIG. 1) is characterised witha constant but in most instances slightly increasing creep rate. InRegion III (indicated by III in FIG. 1) a rapid increase in creep rateoccurs, followed by breaking of the yarn. The creep resistance of theyarn is characterised by the minimum value of the creep rate in theplot, at the start of region II (indicated by A in FIG. 1).

Measurement of Residual Spin Solvent

The chemical type and wt % of solvent is measured via Dynamic headspacegas chromatography coupled to flame ionization detection (FID) and massspectrometry detection in series.

The sample measurement is performed as follows:

The sample is cut in 20 mm pieces. 50 mg of the sample is placed in adesorption tube (thermodesorption system (eg. Gerstel™ TDS 2 with anautosampler Gerstel™ TDS A, both delivered by Sigma-Aldrich Inc.), and acooled injection system (eg. Gerstel™ CIS4, delivered by Sigma-AldrichInc.)).

At each side of the sample the tube is filled over about 30 mm with aplug of glass wool. The tube is heated for 30 minutes at 200° C. Duringheating the sample, a gas-flow of helium, is sent through the tube. Thevolatiles, components which are vaporized by heating, are trapped on acold trap, which is cooled with liquid nitrogen, at −150° C. The coldtrap is heated, after 30 minutes of desorption. The components arecollected in the cooled injection system and injected with a split ratioof 1:20 and separated on a gas chromatographic column (30 m fused silica(id=0.25 mm), CP-Sil™ 8 CB low bleed/MS) using a gas chromatograph (eg.Hewlett Packard™ system type 6890, delivered by Hewlett Packard in theUS).

Detection of the components is performed with a flame ionisationdetector (FID) and a mass selective detector (Hewlett Packard 5973(MSD)).

Determination of the FID peak areas of references and samples areperformed for determination of the type and amount of solvent.

Calculation the response factor (F_(c)) for the n-alkanes using theformula I:

F _(c) =C _(c) /A _(c)  Form. I

Where:

-   A_(c)=peak area of n-alkane measured by the calibration-   C_(c)=concentration of n-alkane in the calibration solution (μg/0.3    μl)    Calculation of the concentration (C_(s)) in mg/kg using formula II:

C _(s) =A _(s) *F _(c) /I _(s)   Form. II

Where:

-   F_(c)=response factor, calculated according tot formula I-   A_(s)=peak area component measured by the sample analysis-   I_(s)=amount of sample (gram)

Comparative Experiment A

Creep and tensile strength of the Dyneema™ SK 75, of which the spinfinish was removed were measured according to the methods given above.The results are given in Table 1. The content of residual spin solventof the yarn was about 400 ppm.

Comparative Experiment B

The Dyneema™ SK 75 yarn of comparative experiment A was kept for 64hours at 135° C. at 1 atmosphere in air. After the treatment creep,tensile strength and residual spin solvent of the yarn were measuredaccording to the methods given above. The results are given in Table 1.The content of spin solvent is well below 100 ppm, however mechanicalproperties of the yarn were deteriorated. It was even impossible toperform the creep test, due to immediate breakage of the yarn.

Comparative Experiment C

The Dyneema™ SK 75 yarn of comparative experiment A was kept for 64hours at 120° C. at 1 atmosphere in air. After the treatment creep,tensile strength and residual spin solvent of the yarn were measuredaccording to the methods given above. The results are given in Table 1.The content of spin solvent is well below 100 ppm, however the tensilestrength of the yarn was deteriorated and the creep level was adverselyaffected.

Experiment 1

The Dyneema™ SK 75 yarn of comparative experiment A was kept for 64hours at 120° C. The yarn was wound around a frame, so that the yarn waskept taut during the treatment. After the treatment creep, tensilestrength and residual spin solvent of the yarn were measured accordingto the methods given above. The content of residual spin solvent waswell below 100 ppm. Further results are given in Table 1. It shows thatafter the treatment (step e) the level of creep is even lower than thelevel of the untreated yarn.

Experiment 2

The Dyneema™ SK 75 yarn of comparative experiment A was put in anautoclave, filled with supercritical CO₂ at a temperature of 100° C. forone hour. The oxygen content of the super-critical CO₂ environment inthe autoclave was 1.2 mol/m³. The yarn was wound around a frame, so thatthe yarn was kept taut during the treatment. After the treatment creep,tensile strength and residual spin solvent of the yarn were measuredaccording to the methods given above. The results are given in Table 1.The content of spin solvent is well below 100 ppm. It shows that afterthe treatment (step e) the level of creep is comparable to that beforethe treatment.

Minimum Tensile creep Example/comparative sttrength rate experimentcN/dtex (1/second) Comp. Exp. A 35 9.0 · 10⁻⁷ Comp. Exp. B 3Undetectable Comp. Exp. C 10 7.0 · 10⁻⁶ Ex. 1 26 7.0 · 10⁻⁷ Ex. 2 30.59.5 · 10⁻⁷

1. A process for removing residual spin solvent from a gel spun, UHMwPEfilament, comprising the steps of: (a) providing a gel spun UHMwPEfilament having a titer of 2-100 dtex, and a residual spin solventcontent of between 100 and 2000 ppm, (b) winding the filament providedin step a) around a frame such that the filament is kept taut with atension of above 0.15 g/dtex, and (c) removing residual spin solventfrom the filament by subjecting the filament around a frame of step b)to an elevated temperature above 80° C. and below 140° C., to obtain atreated UHMwPE filament having a residual solvent level of below 60 ppm.2. The process according to claim 1, wherein the filament provided instep a) has residual spin solvent content of more than 200 ppm.
 3. Theprocess according to claim 1, wherein the filament provided in step a)has a titer of 2-30 dtex.
 4. The process according to claim 1, whereinthe filament is provided in step a) as a multi-filament yarn.
 5. Theprocess according to claim 1, wherein the elevated temperature is above90° C.
 6. The process according to claim 1, wherein the elevatedtemperature is above 100° C.
 7. The process according to claim 1,wherein the elevated temperature is below 130° C.
 8. The processaccording to claim 1, wherein the elevated temperature is below 125° C.9. The process according to claim 1, wherein step (c) is carried out inan environment having an oxygen content of less than 3 mol/m³.
 10. Theprocess according to claim 9, wherein step (c) is carried out under areduced air pressure.
 11. The process according to claim 1, wherein step(c) is carried out for at least half an hour.
 12. The process accordingto claim 1, wherein the treated filament obtained in step (c) has aresidual spin solvent level of below 40 ppm.
 13. The process accordingto claim 1, wherein the treated filament obtained in step (c) has atenacity of at least 3.4 N/tex.
 14. The process according to claim 1,wherein the treated filament obtained in step (c) has a creep rate,measured at 50° C. under a load so that the initial stress is 600 MPa,of less than 5·10⁻⁶ sec−1.
 15. The process according to claim 14,wherein the treated filament has a creep rate of less than 10⁻⁷ sec⁻¹.16. A treated gel spun UHMwPE filament as obtained by the processaccording to claim
 1. 17. A gel spun UHMwPE filament having a diameterof above 16 μm and a residual spin solvent residue of less than 60 ppm.18. The gel spun UHMwPE filament according to claim 17, wherein thefilament has a creep rate, measured at 50° C. under a load so that theinitial stress is 600 MPa, of less than 5·10⁻⁶ sec⁻¹.
 19. The gel spunUHMwPE filament according to claim 17, wherein the filament has a creeprate, measured at 50° C. under a load so that the initial stress is 600MPa, of less than 8·10⁻⁶ sec⁻¹.
 20. The gel spun UHMwPE filamentaccording to claim 17, wherein the filament has a creep rate, measuredat 50° C. under a load so that the initial stress is 600 MPa, of lessthan 10⁻⁷ sec⁻¹.
 21. A multi-filament yarn comprising a filamentaccording to claim
 16. 22. A mono-filament consisting of one filamentaccording to claim
 16. 23. A product for medical application comprisingthe yarn according to claim
 21. 24. A product for medical applicationcomprising the monofilament according to claim 22.